1. Field of Invention
A wide variety of micro-electromechanical devices (MEMS) are known, including accelerometers, DC relay and RF switches, optical cross connects and optical switches, microlenses, reflectors and beam splitters, filters, oscillators and antenna system components, variable capacitors and inductors, switched banks of filters, resonant comb-drives and resonant beams, and micromirror arrays for direct view and projection displays. Though the processes for making the various MEMS devices may vary, they all share the need for high throughput manufacturing (e.g. forming multiple MEMS devices on a single substrate without damage to the microstructures formed on the substrate).
The present invention is in the field of MEMS, and in particular in the field of methods for making micro electromechanical devices on a wafer. The subject matter of the present invention is related to manufacturing of multiple MEMS devices on a wafer, releasing the MEMS structures by removing a sacrificial material, bonding the wafer to another wafer, singulating the wafer assembly, and packaging each wafer assembly portion with one or more MEMS devices thereon, without damaging the MEMS microstructures thereon. More particularly, the invention relates to a method for making a MEMS device where a final release step is performed just prior to a wafer bonding step to protect the MEMS device from contamination, physical contact, or other deleterious external events. A getter or molecular scavenger can be applied to one or both of the wafers before bonding, as can a stiction reducing agent. Except for coating of the MEMS structures to reduce stiction, it is preferred (though not required) that the MEMS structures are not altered physically or chemically (including depositing additional layers or cleaning) between release and wafer bonding.
2. Related Art
As disclosed in U.S. Pat. No. 5,061,049 to Hornbeck, silicon wafers are processed to form an array of deflectable beams, then the wafers are diced into chips, followed by further processing of the individual chips. This process has disadvantages, as disclosed in U.S. Pat. No. 5,445,559 to Gale et al. Once the mirror is formed by etching the sacrificial material to form an air gap between the deflectable beam and a lower electrode, the device is very fragile. The device cannot be exposed to liquids during wafer cleanup steps, without destroying the mirror. “Therefore, the devices must be cut and the dicing debris washed away before etching the sacrificial layer away from the mirror. This requires that the cleaning and etching steps, and any following steps, including testing be performed on the individual chips instead of a wafer.” To address this problem, Gale et al. propose using a vacuum fixture with a plurality of headspaces above the mirrors to prevent contact with the mirrors. The headspaces are evacuated through vacuum ports and the backside of the wafer is ground down to partially sawn kerfs in order to separate the devices. Then the separated devices and the vacuum fixture are washed to remove any debris from the separation operation. The devices with mirrors exposed are finally ready for packaging.
In U.S. Pat. No. 5,527,744 to Mignardi et al., it is likewise desired to avoid damaging the mirror elements when cutting the wafer into individual dies. In Mignardi et al., a partial saw or scribe is performed on the wafer after optionally putting a removable protective coating over the entire wafer to further limit debris from the partial saw or scribe from settling on the mirrors. Then, the protective coating if used and the debris from the partial saw is removed in a post-saw cleaning. Typically the sacrificial layer is then removed, and additional processes may also take place to cover or protect various surfaces of the device that were not exposed previous to removing the sacrificial layer. Last, in order to separate the wafer into individual devices, tape is aligned and applied to the wafer, covering the partially sawed areas. The wafer is broken and the tape is treated with UV light to weaken it and then is peeled away. The individual devices with exposed mirrors must then be carefully picked and placed off of the saw frame and packaged.
U.S. Pat. No. 5,872,046 to Kaeriyama et al., discloses partially fabricating a micromirror structure on a semiconductor wafer, followed by coating the wafer with a protective layer. Then, streets are sawed in the wafer (defining the individual dies), which is followed by cleaning the wafer with a solution of an alkyl glycol and HF. Further processing includes acoustically vibrating the wafer in deionized water. Finally the mirrors are released and the wafer broken along the streets.